Styrene-based resins represented by rubber-reinforcement styrene-based resin have been used in a wide range of fields including domestic electric appliances, office automation machines, and general merchandise, because the styrene-based reins have excellent mechanical properties, molding processability, and appearance trait. Furthermore, heat resistance can be provided to the rubber-reinforcement styrene-based resin by copolymerization with a moderate amount of N-phenyl maleimide represented by an N-substituted maleimide based monomer, and the thus obtained heat resistance resin has been used in car interior and exterior parts and domestic electric appliances.
As a method for copolymerizing such an N-substituted maleimide, a method disclosed in Patent Document 1 has been proposed. However, this method is not able to achieve a homogeneous resin composition distribution, is inferior in color and mechanical strength, and achieves insufficient effect of improvement in heat resistance.
In addition, as a method for obtaining the copolymer with a homogeneous composition distribution, various proposals such as in Patent Document 2 have been made. However, how to add a vinyl-based monomer has only been considered, improvements such as fundamental reexamination of the polymerization method have not been made. In addition, Patent Document 3 discloses a production method in accordance with continuous solution polymerization. However, while this method achieves a homogeneous copolymer composition distribution, the method is disadvantageous in that it is not possible to obtain a copolymer which has high heat resistance and includes less remaining phenyl maleimide. When a monomer containing a large amount of phenyl maleimide is supplied in order to improve the heat resistance, all of the phenyl maleimide is not reacted for polymerization, thus leaving the unreacted phenyl maleimide as remaining phenyl maleimide. Furthermore, the remaining phenyl maleimide causes oligomers to be generated in volatile removing apparatus and causes the composition distribution of the polymer to be made wider. The wider composition distribution of the thermoplastic resin and the remaining phenyl maleimide cause oily substances (bleed) in injection molding. A large amount of bleeding causes notable mold staining, and it is then necessary to stop the automatic production for the purposes of removing the bleed and cleaning the mold, in line with the progress of continuous injection molding, thereby decreasing the efficiency. Besides the remaining phenyl maleimide, residual from a polymerization initiator and a chain transfer agent as polymerization aids also causes a nonhomogeneous composition distribution and an increase in bleeding, and it is thus necessary to minimize the usages of the polymerization aids. However, no reference has been reported in which an approach for minimizing the usages is disclosed.
In addition, the phenyl maleimide monomer itself exhibits a yellow color. Thus, the phenyl maleimide monomer causes coloring of the produced polymer, and there is even a possibility that the phenyl maleimide monomer causes problems in terms of handleability such as scaling or caking in piping during the process for production, because the phenyl maleimide is a solid monomer.
As a method for reducing the remaining phenyl maleimide, it is necessary to progress the polymerization as much as possible for incorporation into the polymer chain. However, the polymer with a high degree of polymerization has poor polymerization stability, and is not able to be produced in a stable manner or an economically advantageous manner.    [Patent Document 1] Japanese Patent Application Laid-Open No. 62-50357    [Patent Document 2] Japanese Patent Application Laid-Open No. 58-162616    [Patent Document 3] Japanese Patent Application Laid-Open No. 61-276807